I was considering a gas weedeater engine for a large tugboat at one time,but decided against it. A gas engine if not water cooled needs air to cool it and even if it's water cooled it needs fresh air to the carb.Another thing to consider is the heat build up in the hull from the exhaust and cylinder heat.Next to use a reverse, if a transmission works, you would need a clutched engine.Clutches engage and disengage at a certain rpm and it would be like reving up you car then putting it in gear.Not a smooth transition from a dead stop to forward or reverse.You would have to use gears or belts and pulleys to gear it down to achieve scale speed.Most large r/c tugs have dual props.You would have to figure how to drive two off one engine.With electric motors you won't have all these problems and if you have a need for sound or smoke there's smoke and sound systems available.Go to the scale boat section here and they can give you advice.